The standard ATX motherboard form factor has been around for a long time (since 1995!), long enough that its layout doesn't really make any sense anymore. We've tried to kill it in the past, to no avail--Dell was the last BTX holdout--but what may finally kill ATX are its smaller siblings. Smaller motherboards allow for smaller cases with more efficient airflow, while still giving you everything you need for a powerful desktop computer. You lose some PCI expansion slots, but how many of those does the typical PC gamer really need?
Valve is clearly betting on a post-ATX world. Its Steam Machine test units measure 12 inches by 12 inches by 3 inches and use standard desktop parts--including, Valve says, replaceable motherboards, so they're almost certainly microATX or mini-ITX based.
ATX vs. MicroATX vs. Mini-ITX
You're probably already familiar with the standard ATX motherboard layout. After all, it's been in most of the desktop computers you've owned for the last 20 years--unless you bought a lot of Dells last decade. You know how it goes: 12 inches high, 9.6 inches wide, I/O ports on the top of the left edge, CPU in the upper middle, RAM to the right of it (and sometimes to the left). SATA ports on the lower right, expansion slots in the lower left quadrant. A typical ATX motherboard has six to eight SATA ports, four or more RAM slots, and seven or eight expansion slots.
I'd argue that many of those port and slots are unnecessary. You can build a fantastic desktop and never need more than two or three storage drives, or more than one graphics card and a sound card. You really only need two RAM slots, thanks to 8GB DIMMs, so why would you put all that into a case that's four cubic feet in volume?
A smaller case with the parts closer together will actually be easier to cool.
A standard ATX case takes in air in the lower part of the front panel, and exhausts it out the top and upper rear of the case. This provides a consistent airflow, but the problem is that the major heat-producing parts--the CPU, GPU, and drives--are not all in a straight line, and they're far enough apart that it's hard to cool all of them efficiently with just a few fans. Hence the giant wind tunnels you see in enthusiast systems with a half a dozen fans. A smaller case with the parts closer together will, counter-intuitively, actually be easier to cool.
MicroATX boards are generally 9.6 by 9.6 inches--just a few inches shorter than ATX motherboards. They tend to have four RAM slots, a full-sized CPU socket, six or so SATA ports, and four or fewer expansion slots. I'd argue that this is the enthusiast sweet spot these days, rather than ATX. A MicroATX system can accommodate a dual-GPU setup or a single GPU and a discrete sound card. If you want both, you'll have to go to full ATX.
Loyd's latest gaming PC build is on a MicroATX board and case, and he had plenty of room for all the components of a powerful gaming computer. He could even have used a mini-ITX board and case, if he didn't want a discrete internal sound card. I'm in exactly the same boat--if it wasn't for the sound card I use to power my headphones, I could be using a mini-ITX board and case too. But then, I already have my MicroATX case and mobo, so there's no reason for me to side-grade. My Sandy Bridge i5-2500K and Z68 motherboard will last me for another few years easily.
Move to Mini-ITX and you do end up with a few size-based constraints. Mini-ITX boards are 6.7 inches squared, and weren't originally designed for full-sized desktop CPU sockets at all. Thankfully, radder heads prevailed and we've been able to buy Mini-ITX boards with Intel and AMD desktop sockets for half of a decade. Mini-ITX boards with desktop sockets tend to use two full-sized RAM slots, have four SATA ports, and one PCIe x16 slot, although some with the Thin Mini-ITX standard use desktop processors but mobile RAM. Thin Mini-ITX boards often have onboard mini-PCIe and mSATA slots for WiFi cards and mini SSDs, but no x16 PCIe slot. These are usually used for very low-profile situations, like all-in-one computers or point-of-sale machines, and they're not really interesting for gamers.
If I were building a system today, and I was using an external sound card or USB speakers like the Bowers & Wilkins MM-1s, I'd use a mini-ITX board and case. Even a great gaming PC really only needs a mobo, CPU, RAM, one graphics card, an SSD, and a mass storage drive. And an optical drive, if you're fancy. A good mini-ITX case, like the BitFenix Prodigy or Silverstone's Fortress FT03-Mini or Sugo SG-08, can easily accommodate a powerful system and cool it efficiently with just one fan.
What's are People Actually Building?
If smaller form factors were really enough for gaming, wouldn't we see mini-ITX and MicroATX mobos and cases, and full-on small form factor gaming rigs, increasing in popularity?
Yep. I asked Tony Ou at Silverstone if he could share some case sales stats with me. In 2010, three of Silverstone's top ten most-sold cases were MicroATX, one was mini-ITX, and the other six were ATX midtowers or full-towers. In 2011, five of the top ten were MicroATX and two were mini-ITX. Last year, the top five were all MicroATX, the 6th and 7th best-selling cases were Mini-ITX, 8 and 9 were MicroATX again, and only the tenth best-selling case was a full ATX case.
Now, those numbers came with some caveats. Tony said, "MicroATX and Mini-ITX have always been major part of our case business so I don’t think we are an accurate indicator for the overall market," and added that SFF cases are more popular in areas with higher incomes, like "North America, Japan, and some parts of Europe," but that the rest of the world is still ruled by full-sized ATX cases. Still, the fact that small form factor cases were the top nine best selling cases for Silverstone shows that there's real demand.
Small-form-factor gaming rigs like the Alienware X51, iBuyPowerRevolt, Digital Storm Bolt, and Falcon Tiki are becoming quite popular. Those cases take the mini-ITX form factor (or, rarely, design their own motherboards) and compress it even further through the use of clever engineering and riser cards. Because they're even smaller than off-the-shelf small form factor cases, these mini gaming rigs tend to cost more than the DIY versions. But man, the small size is appealing. (It's not online yet, but the November 2013 issue of Maximum PC has a four-way roundup of these mini gaming rigs.)
It looks like Valve's Steam Machine, at least in its prototype configuration, will follow the same basic design as the rigs mentioned above. The prototypes will run full-sized GPUs up to the size of Nvidia's Titan, use hybrid hard drives (presumably the 2.5-inch 1TB Seagate Barracuda XT), have 16GB DDR3/1600, run 450W power supplies, and be 12x12.4x2.9 inches--slightly smaller than the Falcon Northwest Tiki.
New Units of Computing
What about folks who use a desktop for the form factor, not the power? Even if I didn't play games, after all, I'd want a 27-inch monitor, keyboard and mouse in my home office. It's just easier to work with that setup than with a 13-inch laptop.
If that's what you want--desktop input/output without the actual desktop--then you have two options. Either get a laptop and a docking station, or a really small form-factor PC.
Honestly, if I wasn't a gamer, I'd get an ultrabook or ThinkPad (or ThinkPad ultrabook) and a docking station so I could connect a nice 27-inch monitor, keyboard and mouse, speakers, and external drive when I was at my desk.
But if I wanted a little more power, or didn't plan on taking my machine with me, ever I'd get one of the newer, even smaller computers that are coming out, like the Intel Next Unit of Computing (NUC). Unlike even tinier and cheaper mini-boards like the Raspberry Pi, the NUC and similar devices like the Gigabyte Brix II use tiny motherboards with full x86 CPUs--usually soldered-on mobile CPUs--and require you to add your own mobile RAM, mini-PCIe WiFi cards, and mSATA SSDs. They're limited to integrated GPUs, but in the case of the Brix II, for example, that means Intel's top-end integrated Iris Pro graphics, which are powerful enough.
Xi3's Piston, clearly designed as some sort of extra-compact Steam Machine, has sort of been revealed as an even smaller alternative to the more traditional Steam Machines of Valve's Hardware Test.
The Old Ways
Obviously, there are still people for whom an ATX full-tower PC makes sense, but they're not the majority of desktop builders and they haven't been for a long time. Most of us really only need one PCIe x16 slot, a couple of RAM slots, a CPU socket, and a few hard drive mounts. If you need multi-GPU setups, a sound card, a RAID card, or tens of terabytes of space--or all of the above--a full ATX board in a full-sized chassis is what you need. But for the rest of us--even most of us--a small-form-factor PC is plenty.